High contrast tends to be difficult to obtain in projection display apparatus, because the light valve in the apparatus receives light that leaks from optical elements in the optical system, as well as stray light (unwanted light) generated by the optical elements, so that dark images are not displayed darkly enough. When images are projected onto a screen in a dark room, if dark images are not displayed darkly enough, the viewer experiences a particularly inadequate sense of contrast. A projection display apparatus has therefore been proposed that uses a first lens array and a second lens array to produce a uniform intensity of light and places a diaphragm mechanism between the first and second lens arrays to adjust the quantity of light incident on the light valve by sliding a diaphragm in a direction parallel to the second lens array or swinging a diaphragm around an axis orthogonal to the optical axis (see, for example, Patent Document 1).
Patent Document 1: WO2005/026835 (pp. 6 and 13, FIGS. 2A, 2B, 11B, and 12)